1. Field of the Invention
The present invention relates to a holding sealing material, a method for manufacturing a holding sealing material and an exhaust gas purifying apparatus.
2. Discussion of the Background
Exhaust gases discharged from internal combustion engines such as a diesel engine contain particulate matter (hereinafter, also referred to as PM). In recent years, the PM has raised serious problems because it is harmful to the environment and the human bodies. Moreover, since exhaust gases also contain toxic gas components such as CO, HC, and NOx, there have been growing concerns about influences of these toxic gas components on the environment and the human bodies.
In view of these, as an exhaust gas purifying apparatus for capturing PM in exhaust gases and for purifying the toxic gas components, various exhaust gas purifying apparatuses have been proposed. Each of those exhaust gas purifying apparatuses includes: an exhaust gas treating body made of porous ceramics such as silicon carbide and cordierite; a casing for housing the exhaust gas treating body; and a holding sealing material disposed between the exhaust gas treating body and the casing. The holding sealing material is installed mainly for purposes of preventing the exhaust gas treating body from being damaged upon contact with the casing that covers its periphery due to vibrations or impacts caused by traveling or the like of an automobile and preventing exhaust gases from leaking between the exhaust gas treating body and the casing.
In order to improve fuel consumption, the internal combustion engines are operated under conditions close to at an air fuel ratio close to the theoretical air fuel ratio. As a result, the temperature of exhaust gases tends to be higher. Especially, when the internal-combustion engine is operated at a high revolution, the temperature of exhaust gases immediately after discharged from the internal combustion engine may rise to as high as about 1000° C. Thus, exhaust gases at a high temperature may reach the exhaust gas purifying apparatus. Therefore, there is a demand for a holding sealing material that is not damaged, that is, for example, a holding sealing material that does not melt even under such a high temperature.
As a holding sealing material intended for use under a high temperature, there has been proposed a holding sealing material including two inorganic fiber layers: a layer containing alumina fibers; and a layer containing inorganic fibers mainly made of alumina and silica (hereinafter also referred to as ceramic fibers) (JP-A 2004-204819).
The holding sealing material described in JP-A 2004-204819 is provided with moderate heat resistance because it includes the two inorganic fiber layers, that is, the layer containing alumina fibers having a relatively high heat resistance and the layer containing ceramic fibers.
When manufacturing an exhaust gas purifying apparatus, a holding sealing material is wound around an exhaust gas treating body, and the exhaust gas treating body with the holding sealing material wound thereon is slid to be press-fitted into a cylindrical casing. Since the outer diameter of the exhaust gas treating body including the thickness of the holding sealing material is larger than the inside diameter of the casing, a great shear stress is loaded on the holding sealing material upon the press-fitting. Therefore, the holding sealing material is required to have a winding ability, which prevents crack and fracture formation upon winding of the holding sealing material, as well as a strength enough to resist the shear stress loaded upon the press-fitting (hereinafter, also referred to as shear strength).
In the mat described in JP-A 2004-204819, an upper layer is formed by forming a second slurry into a sheet on a lower layer obtained by forming a first slurry into a sheet. Patent Document 1 discloses that the slurries are partially mixed between both layers, resulting in formation of an integrated sheet.
The contents of JP-A 2004-204819 are incorporated herein by reference in their entirety.